Metastasis or metastatic disease is the spread of a disease from one organ or part to another non-adjacent organ or part. Metastatic disease is primarily but not uniquely associated with malignant tumor cells and infections (Klein, 2008, Science 321(5897):1785-88; Chiang & Massagué, 2008, New Engl. J. Med. 359(26):2814-23).
Cancer occurs after a single cell in a tissue is genetically damaged in ways that result in the formation of a putative cancer stem cell possessing a malignant phenotype. These cancer stem cells are able to undergo uncontrolled abnormal mitosis, which serves to increase the total number of cancer cells at that location. When the area of cancer cells at the originating site become clinically detectable, it is called primary tumor. Some cancer cells also acquire the ability to penetrate and infiltrate surrounding normal tissues in the local area, forming a new tumor. The newly formed tumor in the adjacent site within the tissue is called a local metastasis.
Some cancer cells acquire the ability to penetrate the walls of lymphatic and/or blood vessels, after which they are able to circulate through the bloodstream (circulating tumor cells) to other sites and tissues in the body. This process is known (respectively) as lymphatic or hematogenous spread. After the tumor cells come to rest at another site, they re-penetrate through the vessel or walls (extravasion), continue to multiply, and eventually another clinically detectable tumor is formed. This new tumor is known as a metastatic (or secondary) tumor. Metastasis is one of the hallmarks of malignancy. Most tumors and other neoplasms can metastasize, although in varying degrees (e.g. basal cell carcinoma rarely metastasize) (Kumar et al., 2005, “Robbins and Cotran Pathologic Basis of Disease”, 7th ed., Philadelphia: Elsevier Saunders).
Metastatic tumors are very common in the late stages of cancer. The most common places for the metastases to occur are the lungs, liver, brain, and the bones. There is also a propensity for certain tumors to seed in particular organs. For example, prostate cancer usually metastasizes to the bones. In a similar manner, colon cancer has a tendency to metastasize to the liver. Stomach cancer often metastasizes to the ovary in women. Breast tumor cells often metastasize to bone tissue. Studies have suggested that these tissue-selective metastasis processes are due to specific anatomic and mechanical routes.
Currently, only six percent of women that are first diagnosed with breast adenocarcinoma present with metastases (Hortobagyi et al., “Neoplasm of the breast”. In: Cancer Medicine. B C Decker; Holland-Frei, Ed., 2006, p. 1584-643). Unfortunately, between twenty and fifty percent of them will eventually develop metastatic disease (Hortobagyi et al., “Neoplasm of the breast”. In: Cancer Medicine. B C Decker; Holland-Frei, Ed., 2006, p. 1584-643) Metastases are responsible for an intolerably high number of deaths among patients that would otherwise be almost invariably cured by surgical resection and adjuvant therapy (Lu et al., 2009, Cancer Res. 69:4951-53). Autopsy studies have estimated that 70% of advanced breast cancer patients have skeletal metastases (Bussard et al., 2008, Cancer Met. Rev. 27:41-55). These secondary bone tumors cause significant morbidity, leading to considerable pain, spinal cord compression and pathological fractures (Coleman, 1997, Cancer 80:1588-94). In addition, when breast cancer cells have disseminated to the skeleton, the resulting bone tumors can be treated only with palliative measures (Body & Mancini, 2002, Off. J. Multi-Natl. Assoc. Supp. Care Cancer 10:399-407; Coleman et al., 2008, Clin. Cancer Res. 14:6387-95; Costa & Major, 2009, Nat. Clin. Pract. Oncol. 6:163-74). The majority of patients develop metastases years after initial treatment of the primary breast tumor. The appearance of late metastases can indeed be attributed to cancer cells disseminated to secondary tissues during different stages of primary tumor progression and remained dormant for variable periods of time. In fact, both early dissemination and dormancy of tumor cells are supported by strong evidence (Huseman et al., 2008, Cancer Cell 13:58-68; Aguirre-Ghiso, 2007, Nat. Rev. Cancer 7:834-46).
Due to the limited size of breast tumors that are first diagnosed today, the vast majority of patients are considered to be viable candidates for breast-conserving surgery (BCS) or lumpectomy. Since BCS minimizes the physical and psychological impact of breast surgery, this approach is widely preferred by patients (Veronesi et al., 2005, Lancet, 365:17271; Morrow, 2009, BMJ 338:b557). In addition, based on studies that reported comparable survival rates between lumpectomy and more radical approaches such as mastectomy, oncologic surgeons are also in favor of this form of treatment (Veronesi et al., 2002, N. Engl. J. Med. 347:1227; Fisher et al., 2002, N. Engl. J. Med. 347:1233). However, the conclusions of these studies are being challenged by a meta-analysis showing that for every four local recurrences prevented, one breast cancer death could be avoided (Clarke et al., 2005, Lancet, 366:2087). In fact, following lumpectomy only 37% of breasts are found tumor-free and between 22% and 59% of patients will need re-intervention because positive or close margins are detected (Sabel et al., 2009, J. Surg. Oncol. 99:99). In addition, reexcision or adjuvant therapies, such as local irradiation or chemotherapy, are normally started several weeks or even months after primary surgery (Buchholz, 2009, N. Engl. J. Med. 360:63; Balduzzi et al., 2010, Cancer Treat. Rev. 36:443) to allow for complete patient's recovery and improve post-operatory aesthetic results.
However, the stroma at the site of tumor removal is characterized by altered angiogenesis, immune cells infiltration and activation of cancer-associated fibroblasts (Hofer et al., “Wound-induced tumor progression: a probable role in recurrence after tumor resection,” Archives of Surgery (Chicago, Ill. 1960), 133, 383, 1998; Stuelten et al., 2008, Cancer Res. 68:7278). These events are potentially able to promote perioperative proliferation and motility of residual cancer cells, thereby allowing their escape into the circulation (Coffey et al., 2006, BioEssays, 28:433; Coffey et al., 2003, Lancet Oncol. 4:760). Even in the presence of dormant cancer cells already lodged into distant sites, the additional spreading of these cells would produce new waves of micrometastases.
The arrest of circulating cancer cells to the skeleton is highly dependent on specific adhesive interactions with the endothelial cells lining the marrow sinusoids (Lehr & Pienta, 1998, J. Natl. Cancer Inst. 90:118-23; Scott et al., 2001, Br. J. Cancer 84:1417; Glinskii et al, 2005, Neoplasia 7:522-27). The required next step is the extravasation of adherent cancer cells drawn by chemo attractant cues generated by the surrounding stroma (Liotta, 2001, Nature 410:24-25). The similarities between cancer cell dissemination and leukocyte trafficking lead to the identification of chemokines as crucial players in both sets of events (Mantovani et al., 2010, Cyt. & Growth Factor Rev. 21:27-39).
The interactions between the chemokine CXCL12 (SDF-1) and its receptor CXCR4 have been extensively studied (Miller et al., 2001, Nature 410:50-56; Dewan et al., 2006, Biomed Pharmacother. 60:273-76). The role of CXCR4 in cell adhesion appears to be dependent on the secondary induction of αvβ3 integrin presentation on the surface of cancer cells and consequent binding to vascular adhesion molecules (Engl et al., 2006, Neoplasia 8:290-301). However, CXCR4 inhibition did not block the binding of colon cancer cells to the liver endothelium, but did limit extravasation (Gassman et al., 2009, Neoplasia 11:651-61). Thus, similarly to its role in hematopoietic stem cells homing, the soluble chemokine CXCL12 seems to be an important player in cancer cell migration into the bone microenvironment rather than mediating adhesive interactions with CXCR4-bearing cells (Gassman, 2008, Chit Exp. Metastasis 25:171-81).
CX3C chemokine receptor 1 (CX3CR1), also known as the fractalkine receptor or G-protein coupled receptor 13 (GPR13), is a protein that in humans is encoded by the CX3CR1 gene (Robertson, 2002, J. Leukoc. Biol. 71(2):173-83; Raport et al., 1995, Gene 163(2):295-99). This receptor binds the chemokine CX3CL1 (also called neurotactin or fractalkine or FKN). FKN is a transmembrane protein that is cleaved into a soluble molecule with potent chemoattractant properties (Bazan et al., 1997, Nature 385:640-44). In its membrane-bound form, FKN can establish strong and stable adhesive interactions with its receptor CX3CR1, and does not require any downstream signaling to induce activation of additional adhesion molecules (Haskell et al., 1999, J. Biol. Chem. 274:10053-58; Imai et al., 1997, Cell. 91:521-30; Goda et al., 2000, J. Immun. 64:4313). Prostate cancer cells were shown to express CX3CR1 and, under dynamic-flow conditions, to adhere to human bone marrow endothelial cells in a FKN-dependent manner (Shulby et al., 2004, Cancer Res. 64:4693-98). In addition, CX3CR1 was shown to be expressed in a high percentage of prostate cancer tissues while human bone marrow supernatants contain soluble FKN, which is released from cells of the bone stroma through a mechanism regulated by androgens (Jamieson et al., 2008, Cancer Res. 68:1715-22). A role for the FKN/CX3CR1 pair in metastasis is also supported by the observation that there is a correlation between CX3CR1 expression in primary breast tumors and clinical metastases. In addition, CX3CR1 expression in pancreatic tumor cells was found to promote the infiltration of the central nervous system (Marchesi et al., 2008, Cancer Res. 68:9060-69). Finally, FKN and CX3CR1 have been recently reported to be involved in adhesion of neuroblastoma cells to the bone in an in vitro system (Nevo et al., 2009, Cancer Lett. 273:127-39).
CX3CR1 was previously detected in the epithelial compartment of normal and malignant human prostate gland tissues (Shulby et al., 2004, Cancer Res. 64:4693; FIG. 1). FKN was also detected both in the soluble fraction of human bone marrow (˜2 ng/ml) and on the surface of human bone marrow endothelial cells (Jamieson et al., 2008, Cancer Res. 68:1715). Human tissue microarrays containing 172 samples of breast cancer were examined, and 131 were found to be positive for CX3CR1 expression (FIG. 2). Because of the high propensity shown by several solid tumors, including breast cancer, to target the skeleton, these results support the idea that functional interactions between FKN in the bone and CX3CR1 on cancer cells are involved in skeletal metastasis. This model was strengthened by the detection of CX3CR1 also in several human breast cancer cell lines; the superior ability of MDA-231 cells to arrest at the skeleton through the blood circulation of mice was related to CX3CR1 expression as compared to other cell lines such as MDA-436 that fail to express this receptor (FIG. 3).
Despite the extensive research on the mechanisms of cancer metastasis, there is not a validated and effective approach to minimize the development of metastasis in patients afflicted with primary tumors. There is a need in the art to identify a method of treatment that efficiently avoids, delays or minimizes the development of metastatic tumors in patients, especially in the context of metastatic bone cancer associated with primary prostate or breast cancers. The present invention fulfills these needs.